Abstract:

Objective: To develop effective dip vat management and waste disposal
strategies, this study focused on establishing ways to stabilise or destabiJise
amitraz, an ectoparasitic compound, in solution. Background: The
formamidines form a small group of insecticides. Their current value lies in
the control of organophosphate and carbamate-resistant pests. The
accumulation of this compound in the environment is of concern because
amitraz is widely used in South Africa to control ticks in mobile and stationary
spray and dip vats of up to 1000 L. Through this process large quantities of
semiconcentrated wastes is generated. Formamidine poisoning symptoms
are distinctly different from other pesticides. Their proposed action is the
inhibition of the enzyme monoamine oxidase, which is responsible for
degrading the neurotransmitters norephedrine and serotonin. Methods: The
pseudo-first order rate constants, k, of 2 ].Lg/ml Amitraz solutions in seven
buffers with a pH range from 3 to 10 and a 0.1 M NaOH solution were
determined at 25°C. The decrease in amitraz concentration was determined
at 285 nm with a spectrophotometer. Using different concentration buffers at
the same pH the effect of buffers on degradation was also tested. The rate of
hydrolysis of amitraz a~ temperatures of 50°C and 75°C were also determined.
This was compared to the results at 25°C. Hydrolysis tempo in different
concentrations of ethanol, propylene glycol and dimethyl sulphoxide (OMSO)
was also determined. The hydrolysis rates in surfactant solutions containing
sodium lauryl sulphate, cetrimide or Tween 80 were also determined. Mass
spectra were used to confirm the hydrolysis products of Amitraz. Results:
Amitraz degrade by means of hydrolysis. At low pH values the acid-stable
2,4-dimethylphenylformamide is formed. This can be further hydrolysed to
2,4-dimethyl aniline. The hydrolysis of 2,4-dimethylphenylformamide is faster
under basic conditions. Thus, the addition of lime, used to stabilise amitraz, will enhance the hydrolysis of its degradation products to aniline. A pseudofirst
order rate process is followed, as described by this equation:
In([amitraz]/[amitraz]o) = -kobst, where [amitraz] is the amitraz concentration at
time t, [amitraz]o is the initial amitraz concentration and kobs is the apparent
pseudo-first order rate constant. At low pH values the hydrolysis of amitraz is
very fast. The hydrolysis rate decreased as the pH rose. It was the slowest at
neutral to alkali pH values. The hydrolysis rate increased again as the pH
values became very high, above pH 10. Hydrolysis was the fastest at 75°C
and the slowest at 25°C. The activation energy for the hydrolysis of amitraz
rose between pH 4 and 6. From pH 6 to 11 the activation energy decreased
at a constant rate. The ionic strength had a slight effect on the hydrolysis of
amitraz in the acetate buffer. At higher ionic strength, the reaction became
slower. Ionic strength had no effect on the phosphate buffer. The pH rate
profile for amitraz hydrolysis was type ABCD, which means that hydrolysis
starts fast and the rate decreased at a constant rate between pH 3 and 6.
The hydrolysis rate decreased further between pH 6 and 10, but this decrease
was slower than that between pH 3 and 6. A small increase in hydrolysis rate
took place between pH 10 and 14. The same ABCD pH rate profile was
observed at 25 and 50°C, but at 75°C the rate of hydrolysis decreased very
slowly between pH 10 and 14. When hydrolysis in three organic solvents
were compared with one another, the propylene glycol solution degraded
amitraz the fastest, ethanol a bit slower and DMSO the slowest. Overalf
degradation was still fastest in water. It is evident for amitraz that anionic
micelles enhance and cationic micelles retard the rate of hydrolysis and that
the magnitude of micellar effects become less with increasing concentrations
of the surfactants. Non-ionic surfactants either decreased or had insignificant
effects on the rate constants for hydrolysis of amitraz. At higher detergent
concentration the catalysis of amitraz hydrolysis became progressively less
pronounced. The maximum rate acceleration occurs in the region of catalyst
concentration at which the bulk of the amitraz is incorporated in the micelles
and additional surfactant, simply solubilise the nucleophllies in the stem layer,
thereby rendering them inactive. Conclusion: It was shown that amitraz
hydrolysis is the fastest in acid conditions and slowest in neutral to alkaline conditions. Hydrolysis also increased with an increase in temperature. Very
importantly, in anionic surfactant solutions amitraz is solubilised and the
hydrolysis rate is increased. This surfactant might therefore be used when
trying to dispose of dip vat waste.